The present invention relates to a process for separating and recovering isotopes, and particularly to a process for separating and recovering hydrogen isotopes suitable for energetically and efficiently removing and enriching hydrogen isotope from liquid water.
A temperature exchange process is available as one of the processes for separating hydrogen isotopes such as deuterium (D) and tritium (T), and the process principally utilizes isotope exchange reactions between the hydrogen compounds. That is, the reactions utilize such an isotope effect that, for example, when water and hydrogen sulfide gas, each at the same deuterium concentration, are mixed together, the deuterium concentration of the water will become higher than that of the hydrogen sulfide gas in the equilibrium state.
For separating and enriching hydrogen isotopes with higher efficiency, a combination of two temperature exchange reactors, the one at a low temperature and the other at a high temperature, i.e. dual temperature process, is used. The exchange reactions to be utilized in the dual temperature process include exchange reactions between water and hydrogen sulfide gas, between water and hydrogen gas, between water and hydrogen bromide, and between ammonia and hydrogen, but the exchange reactions between water and hydrogen sulfide gas and between water and hydrogen gas are regarded as commercially important. The exchange reaction between water and hydrogen sulfide gas has such an advantage that the exchange reaction rate is high enough to avoid the necessity use of a catalyst in the reaction, but has such a disadvantage that the separation coefficient depending upon the chemical equilibrium is small. On the other hand, the exchange reaction between water and hydrogen gas has a large separation coefficient, but a low exchange reaction rate, and thus necessitates use of a catalyst for promoting reaction [Benedict, Pigford: Nuclear Chemical Engineering, P454 (1957), McGraw-Hill, USA].
Recently, development of hydrophobic, highly efficient catalysts for promoting isotope exchange reaction between water and hydrogen gas has progressed, and studies of separating and enriching hydrogen isotopes by dual temperature process between water and hydrogen gas have been extensively made [Canadian Pat. No. 907,292; U.S. Pat. No. 4,025,560; Canadian Journal of Chemistry 50 1900-1906 (1972)].
A flow diagram of the conventional dual temperature process between water and hydrogen gas is shown schematically in FIG. 1 [Benedict, Pigford: Nuclear Chemical Engineering, pp 455-357 (1957) McGraw-Hill, USA], wherein a cold reactor 1 and a hot reactor 2 are filled with hydrophobic catalyst, and countercurrent catalytic reaction takes place between water and hydrogen gas in the catalyst beds. The cold reactor 1 is usually operated at a temperature of 30.degree. C. under one atmosphere, and deuterium is transferred from the hydrogen gas to water under the operating conditions, and heavy water is enriched. On the other hand, the hot reactor 2 is operated at a temperature of 250.degree. C. under about 30 atmospheres. Deuterium is transferred from water to hydrogen gas under the operating conditions, and heavy water is depleted.
However, the reactors are filled with the hydrophobic catalyst in the conventional process, and consequently gas-liquid contact becomes very uneven in the catalyst beds, considerably lowering the reactor efficiency. Furthermore, the operating pressure in the hot reactor is high, and thus there is such a problem as an increased risk of radioactive contamination due to hydrogen gas leakage, particularly tritium leakage when tritium as a hydrogen isotope is handled.
To solve these problems, some of the present inventors made studies and found a process shown in FIG. 2 (U.S. patent application Ser. No. 190,173; Canadian Patent Application No. 360,703), where feed water is made in mists, and then water mists and hydrogen gas are subjected to cocurrent catalytic reaction in the catalyst bed in a cold reactor to make uniform gas-liquid contact and catalytic reaction proceed, and water is vaporized in a hot reactor, and the water vapor and hydrogen gas are subjected to gas-gas cocurrent contact and catalytic reaction therein to make uniform catalytic reaction proceed and also enable the hot reaction to proceed under the atmospheric pressure. However, according to the process, the operation is carried out cocurrently, and thus an operating line depending upon a given material balance and an equilibrium curve depending upon a given chemical equilibrium are crossed, and the multi-stage effect cannot be obtained. Thus, a large number of hot reactors are required for highly enriching the hydrogen isotopes. Each hot reactor 2 is provided with an evaporator 3 and a condenser 4, and thus evaporation and condensation of water are inevitably repeated before and after every hot reactor. Thus, the process shown in FIG. 2 requires a large number of hot reactors and consequently has such a problem that energy consumption will be increased.